The present invention concerns a digital superheterodyne receiver.
It also concerns a baseband filtering method used therein.
Current digital receivers have to extract information on in-phase and quadrature channels after the signal is transposed into the baseband. This is generally achieved using two mixers receiving:
an input signal, and PA0 a signal from a local oscillator for the in-phase channel and the same signal phase-shifted by .pi./2 for the quadrature channel.
The output signal of each mixer has to be filtered (in the analog domain) after which it undergoes analog/digital conversion and then processing in the baseband.
Referring to FIG. 1, for example, a receiver device 30 usually comprises at the input end a first bandpass filter 36, a low-noise amplifier 37 and two channels each comprising a mixer 33, 33', a filter 32, 32' and a sampler 34, 34'. The mixer 33 is connected to a local oscillator via a .pi./2 phase-shifter and the mixer 33' is connected direct to the same oscillator. The respective outputs of the two samplers 34, 34' then undergo digital baseband processing (35). In this type of device the channel filtering is effected at the filters 32, 32' which are implemented in analog technology and whose fixed characteristics strongly condition the overall performance of the receiver and the degree of protection against interference.
This type of processing accordingly has drawbacks in terms of accuracy, drift and adaptation to different channels and different bit rates.